Electrostatic discharge is a naturally occurring electronic phenomenon. Triboelectric charge, often referred to as static electricity, builds up in all materials to some degree, and is eventually discharged as the charge traverses a path towards an electric ground. Static electricity build-up is strongest in insulative materials.
Many electronic components can be easily damaged by electrostatic discharge of relatively small magnitude Magnitudes as small as 50 volts can permanently damage some of these devices, such as microcircuits. For comparative purposes, to illustrate the extreme sensitivity of microcircuits to this phenomenon, a visible charge from a human hand to a doorknob in winter will often exceed 10,000 volts. Thus, extreme caution must be taken in protecting such microcircuit components from electrostatic discharge.
To provide protection from electrostatic discharge for packaged articles, a principle of physics referred to as the Faraday Cage effect is often employed. Electricity does not penetrate a conductive enclosure. The static electric charge will go around the enclosed space, seeking the path of least electrical resistance enroute to ground. By surrounding a static sensitive product with a conductive enclosure, the product is shielded from electrostatic discharge originating outside of the enclosure. However, such Faraday Cage-type packaging, typically, does not protect against static electricity generated internally of the package. Static electricity can be generated when two surfaces rub against each other, as for example when a conductive and a non-conductive surface rub against each other. Specifically, static electricity can be generated internally of a package by an electrical component or other product rubbing against an internal portion of the package or by one portion of the package rubbing against another portion of the package. To protect a product against internally generated static electricity, products are commonly packaged in anti-static (now generally referred to as static-dissipative) packaging material. Such packaging material has a surface resistivity characteristic which prevents a static charge from being built up upon or generated by rubbing against a static-dissipative material. Thus, static-dissipative and electrically conductive packaging materials provide protection against electrostatic charges built up inside and outside a package respectively. More protection is therefore provided when such materials are used in combination rather than separately.
Many products and particularly electrical components such as micro-chips require, in addition to protection against electrostatic discharge, physical protection against damage resulting from impact or contact with outside objects or forces.
It has therefore been an object of this invention to provide a package which provides electrostatic discharge protection as well as physical protection of a packaged product.
Still another object of the present invention has been to provide a package which affords physical protection of the packaged article and which prevents the product from making contact with any surface not made of static dissipative material.
One attempt at protecting static electricity sensitive articles against static electricity is disclosed in U.S. Pat. No. 4,658,985 to McNulty. This patent discloses a bag having two plies of anti-static (polyethylene) material and an electrically conductive fabric or mat embedded therebetween to provide a shield from electrostatic discharge for a bagged article. Because the bag is lined with an anti-static material, it affords protection against electrostatic charge generated internally of the package, but the bag does not, by itself, provide adequate physical protection of the product there enclosed. In order to provide physical protection, the bagged article must be placed within another cell in a rigid container. This results in additional material and material handling costs requiring a bag enclosure and additional labor costs associated with bagging the component or product. Therefore, another object of the present invention is to provide an inexpensive package which not only protects the product or article from electrostatic discharge but also one which provides physical protection of the article.
Two other packages for protecting static electricity sensitive articles or products against static electricity are disclosed in U.S. Pat. No. 4,557,379 to Lane et al and U.S. Pat. No. 4,564,880 to Christ et al. These patents disclose molded plastic clam shell packages made of electrically conductive material for protecting electrostatic sensitive parts, specifically integrated circuit boards and circuits, respectively, between the two halves of the clam shell. Because the dimensions of each package are fixed by the mold design used in manufacturing each package, the variety of product sizes and shapes which each molded package can accommodate is limited. While changes can be made to make these packages somewhat more versatile (for example, changing the mold design to accommodate a product having a different size or shape), such changes add additional costs to already expensive packaging. Thus, it is a further object of the present invention to provide an electrostatic discharge protective package which can be easily and inexpensively made to conform itself to a wide variety of product and article sizes and shapes.
The above-identified U.S. Pat. No. 4,564,880, in addition to providing a package made of electrically conductive material provides an opening on one side of the package which allows part of the enclosed product to be viewed. While this package allows some visual inspection of the enclosed product, the amount of the product which can be inspected through the opening is quite limited (for example, only the printed area on an integrated circuit body). Ideally, most if not all of the product should be visible through the package. Therefore, another object of the present invention has been to provide an electrostatic discharge protective package which provides improved visibility of the enclosed product or article while affording inexpensive electrostatic discharge protection as well as physical protection of the packaged product.